Pulse-echo ultrasonic search units are well known, particularly in nondestructive testing systems; bursts or pulses emitted by such units are detected when reflected back from discontinuities, such as inclusions, as disclosed in Automation Industries' McElroy U.S. Pat. No. 3,821,834, "Method of Making an Ultrasonic Search Unit", granted July 2, 1974. Pulse-echo ultrasonic search units have also been used to measure particles in liquid, as taught in the same company's McGaughey et al. U.S. Pat. No. 3,269,172, "Apparatus for Measuring Particles in Liquids", granted Aug. 30, 1966. A generally similar approach was used to measure discontinuities in blood flowing in a conduit by Patterson et al. ("Microemboli during cardiopulmonary bypass detected by ultrasound", Surg., Gyn. & Obs., 129: 505-510, 1969) and by Szabo et al. ("Arterial blood filter evaluation by echo-ultrasound", Proc. 27th Annual Conference on Engineering in Medicine and Biology, 160: 191, 1974). McGaughey, Patterson et al., and Szabo et al. each of them directed the transmitted signal coaxially longitudinally of the conduit, with a focal point many times the diameter of the conduit in distance from the search unit's transmitting surface. Furthermore, they used flat or convex transmitting surfaces.